Verified answer

G is the gravitational constant

M is the mass of the body that the vehicle will be orbiting

d is the distance between the centers of mass for the body and the vehicle

It works for everything where gravity is the main force that is interacting between the 2 objects

<QUOTE>What is G? The gravitational constant (6.6*10^-11) or 10m/s² ? </QUOTE>

G is the gravitational constant. The capitalization of constants does matter: g (uncapitalized) is the acceleration at the surface of the Earth, i.e. 9.8 m/s^2 (or 10 m/s^2 in your ballpark approximation).

<QUOTE>What is M? The mass of the planet or the mass of the thing?</QUOTE>

That's the mass of the planet/star/whatever is creating the gravitational field which you'll be orbiting.

<QUOTE>What is d? </QUOTE>

That's the distance to the center of mass of the system.

If the body that is being orbited has a mass M much much greater than the mass of the orbiting object (as is the case of a satellite), then for all practical purposes the distance d is the distance to the geometric center of the gravitational field (i.e. the distance to the center of the planet). That's because in that case the center of mass of the system is practically on top of the geometric center of the larger body.

<QUOTE>Does that formula work only for the Earth or for any planet? Does it work for the Moon?</QUOTE>

It works for any planet, star, or moon. M does not distinguish what is the body.

For example, if you imagine that you were very very far away in space, far away from other gravitational fields, you can use that to estimate which velocity you have to put a pencil orbiting around *you*, at a certain distance d from your body's center of mass. It works the same way. As you might expect, you have to imprint a very very tiny velocity to the pencil because your low mass creates a very low gravitational pull.